1. Field of the Invention
The invention relates to a manufacturing method for a hollow rack shaft, and a hollow rack shaft.
2. Description of the Related Art
In an existing steering system, rotation of a steering shaft resulting from steering operation is converted to reciprocating movement of a rack shaft by a rack and pinion mechanism to thereby change the steered angle of steered wheels, that is, the traveling direction of a vehicle. In such a steering system, in recent years, what is called hollow rack shaft that is obtained by forming a rack shaft in a hollow shape is used in order to reduce the weight, or the like.
As a manufacturing method for such a hollow rack shaft, there is known a manufacturing method in which a tubular member is used as a raw martial and the tubular member is plastically deformed to form rack teeth. For example, Japanese Patent Application Publication No, 59-92141 (JP-A-59-92141) describes a method in which a first mandrel having a plurality of protrusions and a substantially semi-cylindrical second mandrel are inserted into a tubular member and then the tubular member is subjected to pressing using a die that has a tooth-shaped portion substantially symmetrical with rack teeth to thereby form the rack teeth to manufacture a rack shaft.
However, in the manufacturing method for a hollow rack shaft, described in JP-A-59-92141, as shown in FIG. 17, the tooth-shaped portion 72 formed in the die 71 is pressed against the tubular member 75 in which the first mandrel 73 and the second mandrel 74 are inserted to thereby form rack teeth. That is, by pushing portions that become groove portions of the rack teeth toward an inner surface 75b of the tubular member 75, the rack teeth are formed so that portions that become tooth portions of the rack teeth are higher than the groove portions. Thus, portions of wall move from the side of an outer surface 75a of the tubular member 75 toward the inner surface 75b and therefore, as shown in the drawing, portions of wall swell at the side of the inner surface 75b of the tubular member 75, but portions of wall do not swell at the side of the outer surface 75a of rack teeth. Thus, it is difficult for each tooth portion to have a large height. Therefore, it is difficult to press portions that become tooth tips of the rack teeth against the grooves 72a of the tooth-shaped portion 72 with a sufficient force, so that there is a possibility that the shape of the rack teeth cannot be accurately formed.